Re: [PATCH v2 3/6] mm/sparse-vmemmap: Fix DAX vmemmap accounting with optimization

Mike Rapoport
2026-04-15T15:59:02Z

On Wed, Apr 15, 2026 at 07:14:09PM +0800, Muchun Song wrote:
> When vmemmap optimization is enabled for DAX, the nr_memmap_pages
> counter in /proc/vmstat is incorrect. The current code always accounts
> for the full, non-optimized vmemmap size, but vmemmap optimization
> reduces the actual number of vmemmap pages by reusing tail pages. This
> causes the system to overcount vmemmap usage, leading to inaccurate
> page statistics in /proc/vmstat.
>
> Fix this by introducing section_vmemmap_pages(), which returns the exact
> vmemmap page count for a given pfn range based on whether optimization
> is in effect.
>
> Fixes: 15995a352474 ("mm: report per-page metadata information")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
> mm/sparse-vmemmap.c | 32 ++++++++++++++++++++++++++++----
> 1 file changed, 28 insertions(+), 4 deletions(-)
>
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 40290fbc1db4..05e3e2b94e32 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -652,6 +652,29 @@ void offline_mem_sections(unsigned long start_pfn, unsigned long end_pfn)
> }
> }
>
> +static int __meminit section_vmemmap_pages(unsigned long pfn, unsigned long nr_pages,
> + struct vmem_altmap *altmap,
> + struct dev_pagemap *pgmap)
> +{
> + unsigned int order = pgmap ? pgmap->vmemmap_shift : 0;
> + unsigned long pages_per_compound = 1L << order;
> +
> + VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, min(pages_per_compound,
> + PAGES_PER_SECTION)));
> + VM_WARN_ON_ONCE(pfn_to_section_nr(pfn) != pfn_to_section_nr(pfn + nr_pages - 1));
> +
> + if (!vmemmap_can_optimize(altmap, pgmap))
> + return DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE);
> +
> + if (order < PFN_SECTION_SHIFT)
> + return VMEMMAP_RESERVE_NR * nr_pages / pages_per_compound;
> +
> + if (IS_ALIGNED(pfn, pages_per_compound))
> + return VMEMMAP_RESERVE_NR;
> +
> + return 0;
> +}
> +
> static struct page * __meminit populate_section_memmap(unsigned long pfn,
> unsigned long nr_pages, int nid, struct vmem_altmap *altmap,
> struct dev_pagemap *pgmap)
> @@ -659,7 +682,7 @@ static struct page * __meminit populate_section_memmap(unsigned long pfn,
> struct page *page = __populate_section_memmap(pfn, nr_pages, nid, altmap,
> pgmap);
>
> - memmap_pages_add(DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE));
> + memmap_pages_add(section_vmemmap_pages(pfn, nr_pages, altmap, pgmap));
>
> return page;
> }
> @@ -670,7 +693,7 @@ static void depopulate_section_memmap(unsigned long pfn, unsigned long nr_pages,
> unsigned long start = (unsigned long) pfn_to_page(pfn);
> unsigned long end = start + nr_pages * sizeof(struct page);
>
> - memmap_pages_add(-1L * (DIV_ROUND_UP(nr_pages * sizeof(struct page), PAGE_SIZE)));
> + memmap_pages_add(-section_vmemmap_pages(pfn, nr_pages, altmap, pgmap));
> vmemmap_free(start, end, altmap);
> }
>
> @@ -679,9 +702,10 @@ static void free_map_bootmem(struct page *memmap, struct vmem_altmap *altmap,
> {
> unsigned long start = (unsigned long)memmap;
> unsigned long end = (unsigned long)(memmap + PAGES_PER_SECTION);
> + unsigned long pfn = page_to_pfn(memmap);
>
> - memmap_boot_pages_add(-1L * (DIV_ROUND_UP(PAGES_PER_SECTION * sizeof(struct page),
> - PAGE_SIZE)));
> + memmap_boot_pages_add(-section_vmemmap_pages(pfn, PAGES_PER_SECTION,
> + altmap, pgmap));
> vmemmap_free(start, end, NULL);
> }
>
> --
> 2.20.1
>

--
Sincerely yours,
Mike.